1. Field of the Invention
The present invention is related generally to a distributed control system for an electric power system and, more specifically, to a distributed control system which utilizes a plurality of independent control elements with each of the control elements having the capacity to independently respond to the magnitude of a sensed parameter at a time determined by a random value.
2. Description of the Related Art:
It is known to provide an electric circuit with a plurality of corrective components, such as resistors, capacitors or inductors, which are connectable in electrical communication with the electric circuit for the purpose of responding to the magnitude of a sensed parameter, such as a voltage potential at a predetermined point in the electrical circuit, an electrical current or power factor. The plurality of corrective components is provided for two purposes. First, the use of a plurality of corrective components provides needed redundancy in situations where long term use of the control system may result in the failure of one or more of the corrective components or their associated control elements at a time when repair is inconvenient or impossible. In addition, the plurality of corrective components provides a degree of resolution in the corrective response to a sensed condition relating to the electrical circuit. For example, a minor correction can be achieved by activating very few corrective components, whereas a major correction may require many or all of the available corrective components to be activated.
U.S. Pat. No. 4,334,256, which issued to Mings on June 8, 1982, describes an automotive surge suppressor which provides a high power semiconductor switching arrangement that is controlled by a sensing circuit to suppress transient surges while maintaining operating voltage. The magnitude of a surge determines the rate of current flow through the semiconductor arrangement to maintain the operating voltage required while dissipating the excess power across a load resistance. The surge suppressor is intended to protect an electrical system and is particularly applicable to automotive electrical systems.
U.S. Pat. No. 4,514,619, which issued to Kugelman on Apr. 30, 1985, describes a circuit which provides indirect current monitoring via voltage and impedance monitoring. It includes electrical circuitry for monitoring the current flow through resistance elements on the leading edge of an aircraft wing or a horizontal stabilizer. The resistance elements are individually and sequentially activated for heating to thereby effect a de-icing or anti-icing function. The non-activated elements are sensed by the circuitry to determine inferentially if there is a predetermined current flow through the heating elements to achieve the intended de-icing or anti-icing function.
U.S. Pat. No. 4,740,859, which issued to Little on Apr. 26, 1988, describes a transient surge suppressor and line short monitor. The device is utilized in an alternating current transmission system and includes a metallic case or casing into which lead lines connected to each of the power lines and to the neutral lines are introduced. The metalic case encompasses a series connected varistor and fuse pair connected between each lead from an incoming alternating current power line and the lead from the neutral line. A series connected neon lamp and limiting resistor pair inside of the case are connected across each varistor to be normally energized to give visual indication of the effectiveness and readiness of the suppressor to pass voltage surges above a predetermined maximum voltage to neutral and to give visual indication that there is no short circuit between the power lines and the neutral line.
U.S. Pat. No. 4,428,021, which issued to Chen et al on Jan. 24, 1984, discloses a current monitoring apparatus for monitoring the fault current in the ground conductor of a polyphase alternating current electrical power system. When the fault current exceeds a predetermined level, the magnetic field produced causes a magnetic reed switch to close and open at twice the alternating current frequency. Pulse producing circuitry connected to the reed switch responds to the switch action to produce pulses at the same frequency. The pulses are counted by logic circuitry and when a predetermined number of pulses have been counted an output signal triggers circuitry which trips circuit breaker apparatus in the power system. The delay period between first detecting the fault current and tripping the circuit breaker apparatus can be adjusted by changing the number of pulses to be counted by the logic circuitry before it produces an output signal.
When a control system incorporates a plurality of control elements, with each control element having the capability of contributing to an effort to correct a characteristic of an electrical circuit, some scheme must be employed to coordinate the action of each of the control elements. If a central controller is utilized, this coordination is relatively easy to implement. The central controller merely determines a sequence of activation for each of the corrective components along with a predetermined time delay between subsequent activations. However, many applications do not lend themselves to the use of a single central controller. For example, in space flight applications, redundancy must be provided to assure that a failed controller does not jeopardize the mission. If a single central controller is utilized and that central controller fails for any reason, the successful operation of the control system is deleteriously affected. To avoid this problem, a plurality of independent control elements can be used, with each independent control element controlling an individual corrective component such as a resistor, a capacitor, an inductor or any other electrical component that can be utilized to correct a sensed parameter relating to the electrical circuit.
The use of independent control elements introduces another problem. If each of the independent control elements responds to a sensed parameter without coordination with the other independent control elements, some means must be provided to prevent all of the independent control elements from actuating their respective corrective components simultaneously. If this occurs, a severe overcorrection could result, making the control system unstable. In other words, when a monitored parameter exceeds an acceptable threshold magnitude, all of the independent control elements could react simultaneously to reduce that sensed parameter, such as voltage, with the possibility that the overcorrection could result in a severe reduction in the sensed parameter. That severe reduction could reduce the sensed parameter to a magnitude below an acceptable threshold and subsequently induce the independent control elements to once again react in the opposite direction, again over compensating. It should be apparent that some means of coordination to prevent this instability must be provided.
One method that can be employed to avoid the unstable situation described above is to provide each independent control element with a predetermined time delay. In this type of system, each independent control element would delay its response by its assigned predetermined time delay and only activate its respective corrective component after the time delay period has elapsed and the sensed parameter is again measured. If each of the independent control elements is assigned a different time delay, they would respond sequentially to a sensed parameter that requires correction and the magnitude of the sensed parameter would be corrected over a period of time as each of the independent control elements progressively activates its associated corrective component. Therefore, when the magnitude of a sensed parameter is determined to be out of an acceptable range, each of the independent control elements would sense this condition. Then, the independent control element with the shortest time delay would activate its respective corrective component first. Subsequently, the independent control element with the second lowest time delay would activate its corrective component if the magnitude of the sensed parameter remained out of the acceptable range. Subsequently, the independent control element with the third lowest time delay would activate its corrective component following a remeasurement of the sensed parameter. This process would continue until the magnitude of the sensed parameter once again achieved an acceptable level. This technique solves the problem described above relating to the simultaneous activation of all of the independent control elements. However, this technique also introduces another disadvantageous characteristic. For example, if the independent control element with the lowest time delay value fails for any reason, the speed of response of the control system is slowed by that time delay value. If the control system is employed in a spacecraft, or any other apparatus in which replacement of defective parts is difficult, the response of the control system is forever slowed. It should be apparent that, in a system as described above, if two or more of the independent control elements with the shortest time delays have failed, a significant degradation of the control system will be experienced. In addition, this degradation of response will remain in effect during the entire mission of the apparatus in which this type of control system is utilized.
A significant benefit can be achieved if a control system is provided in which independent control elements are utilized, but wherein a technique is employed to avoid the response degradation which is encumbent when the independent control units with the shortest fixed time delays fail. Furthermore, if a plurality of independent control elements is provided in the control system and the control elements are employed in reaction to a sensed parameter in some order which is not constantly repeated, the magnitude of the sensed parameter can be corrected in an orderly fashion and the failure of one or more of the control elements will not have the continuous response degradation effect described above.